It is well known that phyllosilicates, such as smectite clays, e.g., sodium montmorillonite and calcium montmorillonite, can be treated with organic molecules, such as organic ammonium ions, to intercalate the organic molecules between adjacent, planar silicate layers, for bonding the organic molecules with a polymer, such as an epoxy resin, for intercalation of the polymer between the layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The thus-treated, intercalated phyllosilicates, having interlayer spacings of at least about 10-20 .ANG. and up to about 300 .ANG., then can be exfoliated, e.g., the silicate layers are separated, e.g., mechanically, by high shear mixing. The individual silicate layers, when admixed with a matrix polymer, e.g., an epoxy--see U.S. Pat. Nos. 4,889,885; 5,554,670; 5,760,106 and 5,801,216 --have been found to improve one or more properties of the polymer, such as mechanical strength and/or high temperature characteristics.
Exemplary prior art composites, also called "nanocomposites", are disclosed in published PCT disclosure of Allied Signal, Inc. WO 93104118 and U.S. Pat. No. 5,385,776, disclosing the admixture of individual platelet particles, derived from intercalation of layered silicate materials, with a polymer to form a nanocomposite having one or more properties of the matrix polymer improved by the addition of the exfoliated intercalate. As disclosed in WO 93/04118 and U.S. Pat. No. 5,554,670, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer. Such quaternary ammonium cations are well known to convert a highly hydrophilic clay, such as sodium montmorillonite or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules.
In accordance with the present invention, intercalates are prepared by contacting a phyllosilicate with an onium ion that does not contain an epoxy-reactive functional moiety so that a subsequently intercalated anhydride-cured epoxy does not bond to the intercalated onium ion molecule.
In accordance with an important feature of the present invention, best results are achieved by mixing the layered material with the onium ion molecules in a molar ratio of at least about 0.5:1 onium ions to layered material interlayer exchangeable cations, preferably at least about a 1:1 molar ratio of onium ions to exchangeable platelet cations. Regardless of the concentration of onium ion spacing/coupling agent, the intercalating composition should have an onium ion layered material weight ratio of at least 1:20, preferably at least 1:10, more preferably at least 1:5, and most preferably at least about 1:4 to achieve sufficient ion-exchange of the protonated atom of the onium ion molecule (N.sup.+, P.sup.+, O.sup.+ or S.sup.+) with inner surface cations of the platelets of the layered material to achieve efficient intercalation and bonding of the onium ion compound at the platelet surfaces for subsequent exfoliation, particularly after epoxy resin intercalation. The onium ion intercalant compound sorbed between and bonded to (ion-exchanged with) the silicate platelets causes sufficient separation or added spacing between adjacent silicate platelets for easy co-intercalation of the anhydride-curable epoxy resin. It should be understood that when determining the molar ratio of onium ions to layered material, it is the moles of onium ion (without the dissociated anion) that is calculated, without considering the molecular weight of the dissociated anion once the onium ion compound is solubilized.
One prior art method of preparing layered silicate-epoxy nanocomposites is disclosed by Giannelis in U.S. Pat. No. 5,554,670. In accordance with the method disclosed in the Giannelis '670 patent, a smectite-type clay is first contacted with an organic compound containing alkylammonium ions having functional groups which are reactive to epoxy resin molecules. The clay layers are attached to the polymer network via onium ion-exchange with the clay platelet cations--the epoxy resin reacting with the reactive functionality on the onium ion molecule. The nanocomposites disclosed in the '670 patent exhibit a slightly increased glass transition temperature--the dynamic storage modulus of the nanocomposite was considerably higher in the glassy region and very much higher in the rubbery region when compared with the pristine matrix polymer.
It has now been found that the glass transition temperature (Tg) of rigid anhydride-cured epoxy resins (rigid being defined herein as having a Tg&gt;30.degree. C.) can be unexpectedly raised by mixing the anhydride-curable epoxy resin with a nanomer formed by intercalating a layered silicate material, such as a phyilosilicate, with organic onium ions to space the adjacent platelets sufficiently for intercalation of an anhydride-curable epoxy resin, and mixing the onium ion-intercalated layered material with an anhydride-curable epoxy resin and an anhydride curing agent to co-intercalate the anhydride-curable epoxy resin with onium ions and the anhydride curing agent to form the nanocomposite composition.
In principle, the anhydride-curable epoxy resin and onium ion co-intercalants of the present invention perform together in the gallery of the layered materials to make the inorganic layered materials compatible with the epoxy matrix polymer and form the nanocomposite. The anhydride-cured epoxy resin does not bond to the onium ions or to the phyllosilicate platelets since the onium ions are not epoxy-reactive functionalized. The co-intercalates made by the process of the present invention can be admixed with all market available epoxy resin systems to form nanocomposites. Examples of suitable epoxy resins include: Bisphenol A-derived resins, Epoxy cresol Novolac resins, Epoxy phenol Novolac resins, and the like.